Figure 1: MODIS aboard the Aqua satellite captures a combined visible-infrared imagery (with corrected reflectance, resolution of 250m) of the Great Lakes region on February 20, 2021. Source: NASA Earth Observatory.
After an abnormally warm start to winter, a recent cold air outbreak affecting a large swath of the interior United States has contributed to a noticeable spike in ice coverage across most of the Great Lakes. The image above was acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite on February 20, 2021. This false-color image uses a combination of shortwave infrared, near-infrared, and visible bands that help distinguish ice coverage from snow, liquid water and clouds. Sea ice is denoted in the pale blue regions within the major bodies of water (dark blue to black), and is in its highest concentrations in western Lake Superior, Green Bay (northwest of Lake Michigan), and underneath the cloud cover over Lake Erie.
According to measurements taken by the Great Lakes Environmental Research Laboratory (GLERL), only 2.81% of the Great Lakes were ice-covered to usher in 2021, and increased to 10.65% by February 1, 2021. In an average year, between 20 and 40 percent of these bodies of water would contain ice by February 1 before reaching its respective peak in ice coverage by late February. By February 20, 2021, total ice coverage across the Great Lakes jumped to 44.71%, which brought ice concentrations to near-average totals for this time of year. Compared to the 2019-2020 winter season, a similar lack of cold air throughout the season kept the maximum ice coverage under 20% the entire season, which was below the average ice concentration for the Great Lakes.
Figure 3: Loop of mean sea level pressure (MSLP) over the Great Lakes from 00Z 13 February 2021 to 00Z 19 February 2021 using a Rapid Refresh Reanalysis product. Higher values of MSLP (high pressure systems) are denoted in brighter colors, whereas lower values (low pressure systems) are denoted in cooler colors.
What’s helping to finally bring this cold air over the Great Lakes? Beginning on February 6, 2021, a series of high pressure systems would originate over central Canada and the northern Great Plains, with the most potent of these systems affecting all of the Great Lakes beginning on February 13, 2021. In a high pressure system, also known as an anticyclone, the air around the system flows in a clockwise pattern in the Northern Hemisphere. If an anticyclone is located to the west of a region, as was the case throughout the majority of the week, cold air will enter the region from the north, and will continue to be pumped into the region until the upper level dynamics either weakens the high pressure system or moves it out of the area. At its peak, the latest high pressure system over the northern Great Plains reached a mean sea level pressure of 1046 millibars on February 14, 2021, which corresponded to low temperatures ranging from -25 degrees to +10 degrees Fahrenheit along the shores of the Great Lakes, with lower wind chill values. As of February 20, 2021, this high pressure system will begin to move eastward, which will allow for warmer temperatures to enter the region through the early portions of next week, although not warm enough for ice formation to decline just yet.